Communication systems designed to incorporate the characteristic of communicating with many remote subscriber units for brief intervals on the same communication channel are termed multiple access communication systems. One type of communication system which can be a multiple access system is a spread spectrum system. In a spread spectrum system, a modulation technique is utilized in which a transmitted signal is spread over a wide frequency band within the communication channel. The frequency band is much wider than the minimum bandwidth required to transmit the information being sent. A voice signal, for example, can be sent with amplitude modulation (AM) in a bandwidth only twice that of the information itself. Other forms of modulation, such as low deviation frequency modulation (FM) or single sideband AM, also permit information to be transmitted in a bandwidth comparable to the bandwidth of the information itself. However, in a spread spectrum system, the modulation of a signal to be transmitted often includes taking a baseband signal (e.g., a voice channel) with a bandwidth of only a few kilohertz, and distributing the signal to be transmitted over a frequency band that may be many megahertz wide. This is accomplished by modulating the signal to be transmitted with the information to be sent and with a wideband encoding signal.
Generally, three types of spread spectrum communication techniques exist, including:
Direct Sequence
The modulation of a carrier by a digital code sequence whose bit rate is much higher than the information signal bandwidth. Such systems are referred to as "direct sequence" modulated systems.
Hopping
Carrier frequency shifting in discrete increments in a pattern dictated by a code sequence. These systems are called "frequency hoppers." The transmitter jumps from frequency to frequency within some predetermined set; the order of frequency usage is determined by a code sequence. Similarly "time hopping" and "time-frequency hopping" have times of transmission which are regulated by a code sequence.
Chirp
Pulse-FM or "chirp" modulation in which a carrier is swept over a wide band during a given pulse interval.
Information (i.e. the message signal) can be embedded in the spread spectrum signal by several methods. One method is to add the information to the spreading code before it is used for spreading modulation. This technique can be used in direct sequence and frequency hopping systems. It will be noted that the information being sent must be in a digital form prior to adding it to the spreading code, because the combination of the spreading code and the information, typically a binary code, involves module-2 addition. Alternatively, the information or message signal may be used to modulate a carrier before spreading it.
Thus, a spread spectrum system must have two properties: (1) the transmitted bandwidth should be much greater than the bandwidth or rate of the information being sent and (2) some function other than the information being sent is employed to determine the resulting modulated channel bandwidth.
Spread spectrum communication systems can be implemented as multiple access systems in a number of different ways. One type of multiple access spread spectrum system is a code division multiple access (CDMA) system. CDMA spread spectrum systems may use direct sequence (DS-CDMA) or frequency hopping (FH-CDMA) spectrum spreading techniques. FH-CDMA systems can further be divided into slow frequency hopping (SFH-CDMA) and fast frequency hopping (FFH-CDMA) systems. In SFH-CDMA systems, several data symbols representing a sequence of data bits to be transmitted modulate the carrier wave within a single hop; in FFH-CDMA systems, the carrier wave hops several times per data symbol.
In a SFH-CDMA system, multiple communication channels are accomodated by the assignment of portions of a broad frequency band to each particular channel. For example, communication between two communication units in a particular communication channel is accomplished by using a frequency synthesizer to generate a carrier wave in a particular portion of a predetermined broad frequency band for a brief period of time. The frequency synthesizer uses an input spreading code to determine the particular frequency from within the set of frequencies in the broad frequency band at which to generate the carrier wave. Spreading codes are input to the frequency synthesizer by a spreading code generator. The spreading code generator is periodically clocked or stepped through different transitions which causes different or shifted spreading codes to be output to the frequency synthesizer. Therefore, as the spreading code generator is periodically clocked, the carrier wave is frequency hopped or reassigned to different portions of the frequency band. In addition to hopping, the carrier wave is modulated by data symbols representing a sequence of data bits to be transmitted. A common type of carrier wave modulation used in SFH-CDMA systems is M-ary frequency shift keying (MFSK), where k=log.sub.2 M data symbols are used to determined which one of the M frequencies is to be transmitted.
Multiple communication channels are allocated by using a plurality of spreading codes to assign portions of the frequency band to different channels during the same time period. As a result, transmitted signals are in the same broad frequency band of the communication channel, but within unique portions of the broad frequency band assigned by the unique spreading codes. These unique spreading codes preferably are orthogonal to one another such that the cross-correlation between the spreading codes is approximately zero. Particular transmitted signals can be retrieved from the communication channel by despreading a signal representative of the sum of signals in the communication channel with a spreading code related to the particular transmitted signal which is to be retrieved from the communication channel. Further, when the spreading codes are orthogonal to one another, the received signal can be correlated with a particular spreading code such that only the desired signal related to the particular spreading code is enhanced while the other signals are not enhanced.
As CDMA technology becomes incorporated into next generation cellular systems, practical system complications due to the nature of cellular systems arise. For example, in cellular systems incorporating soft handoff, transmitted frame synchronization is critical to proper operation. During soft handoff, a mobile having diversity reception capability receives voice or control transmissions from two base. Depending on the strength, or quality, of the transmission by either base-station, the mobile will choose the transmission of the base-station having the best signal quality. This configuration of the cellular system requires that the two base-stations transmit the same voice or control data at the same time so that the mobile could perform diversity on both signals from both base-stations.
The data packet synchronization process should also keep the packet delay as minimum as possible with respect to the air framing boundaries in order to reduce the overall packet delay in the system. The synchronization process is inherently made more difficult since base-stations are typically at different distances from a central data distribution point (perhaps a switch). For accurate synchronization of transmission, the difference in distance of the links or trunks connecting the central data distribution points to the base-stations needs to be accounted for. Typical methods compute the delay from the central data distribution point to each base-station and accordingly delay the data packet to be transmitted. This process, however, has several major drawbacks. First, the computation of delay is a intensive calculation or measurement which consumes valuable processor time during handoff. More importantly, the delay difference between the cells and the central data distribution point is in the magnitude of several hundreds of .mu.seconds. Considering a real-time processing environment for processing the packet delay computation (stamping dummy outbound packets and monitoring base-station arrival time messages), the computation response uncertainty is approximately the same magnitude as the measurement objectives. This results in a need to add one full packet delay (20 msec) in a case of mis-match caused by uncertainty in the computation response.
Thus, a need exists for a data packet alignment scheme which provides a finer degree of packet alignment resolution and is also not computation intensive.